As per the linked FAQ,
What platforms will Sid Meier’s Civilization VII be available on?
Sid Meier’s Civilization VII will be available on PS5, PS4, Xbox Series X|S, Xbox One, Nintendo Switch, and PC via Steam (which also supports Mac and Linux) and the Epic Games Store.
The Steam store (preorder) listing shows all three platform icons so I’m guessing they’re not just talking about making it work with Proton.
No mention yet of Aspyr (who ported Civ 5 & 6) as far as I can see.
They’re just mean via proton, which tbh is fine. Everything works great through proton.
I dunno. There’s no Proton on Mac, so lumping Mac and Linux together when they really just mean Proton wouldn’t make much sense. There are native Mac and Linux games on Steam, and the Steam store page has icons and system requirement tabs for all three OSes, neither of which happens with Windows-only games. “We’re also releasing Civilization VII for MacOS and Linux / SteamOS” also sounds like they’re talking about actual Linux and Mac builds. It could all be miscommunication, of course. That wouldn’t surprise me.